Since thermotropic liquid-crystalline polymer uses expensive aromatic monomers as raw material to result in high price of the polymer, cut-down of manufacturing cost thereof becomes an important problem. As one of cut-down manners of manufacturing cost, attainment of high yield of the polymer is considered.
As for technique of attaining high yield of the polymer, there is such manner that, after termination of polymerization reaction, temperature of a reaction pot is raised at discharging the polymer to lower molten viscosity of the polymer. However, the polymer is thermally degraded to lead to discoloration and generation of blackish brown polymer and, when thermal degradation is heavy, gas of low boiling components derived from raw material (such as phenol (PhOH), benzoic acid (BA), 4-hydroxybenzoic acid (HBA) or phenol ester compound thereof (HBA-Ph)) or black speck (substance like carbide, abbreviated to BS) generates to give an adverse effect on product quality. Further, there is such problem that, when gas of low boiling components fills in a reaction pot, discharge becomes unstable to lower collection rate from the reaction pot.
Among low boiling point components, in particular components relating to phenol (PhOH, HBA-Ph) are caused by phenol generating by thermal degradation of 4 -hydroxybenzoic acid (HBA) residue that is generally used for thermotropic liquid-crystalline polymer. Especially, when acylation-deacylation is conducted in one pot to perform manufacture, degradation (decarboxylation reaction) from HBA to phenol occurs in no small way from the early stage of polymerization. Accordingly, in a liquid-crystalline polymer in which HBA is the main component, a technique is expected for inhibiting decarboxylation reaction of HBA component during polymerization.
In order to inhibit decarboxylation reaction, a method is proposed (see JP-A-2-153922), in which aromatic diol is acylated followed by reaction with HBA to give oligomer, which is subjected to polycondensation with aromatic dicarboxylic acid. However, it can not be applied to a skeleton in which hydroxycarboxylic acid occupies most part. Further, it cannot be said that inhibition of conversion of HBA into phenol is sufficient. Use of an excess acylating agent to a hydroxyl group or an amino group tends to inhibit decarboxylation reaction. However, by this manner, discoloration of polymer or thickening due to side reaction becomes significant. The present applicant proposed previously a method in which generation of low boiling point gas is inhibited by maintaining a charge amount of a raw monomer, an amount of an acylating agent and an amount of a catalyst in a specified relationship to obtain a high quality polymer without discoloration due to thermal degradation and the like at high yield (see JP-A-2002-363281). However, since the metal salt catalyst used for accelerating acylation, basically, also has an effect of accelerating decarboxylation reaction, the inhibition effect had a limit.